Phases In Duplex Stainless Steel Research Articles

Detecting the Sigma Phase in Duplex Stainless Steel by Magnetic Noise and First Harmonic Analysis.

Non-destructive electromagnetic tests based on magnetic noise analysis have been developed to study, among others, residual stress, heat treatment outcomes, and harmful microstructures in terms of toughness. When subjected to thermal cycles above 550 °C, duplex stainless steels form an extremely hard and chromium-rich constituent that, if it is superior to 5%, compromises the steel's corrosion resistance and toughness. In the present work, a study was carried out concerning the interaction of excitation waves with duplex stainless steel. Hence, by analyzing the magnetic noise and variations in the amplitude of the first harmonic of the excitation waves, the detection of the deleterious sigma phase in SAF 2205 steel is studied. To simplify the test, a Hall effect sensor replaced the pick-up coil placed on the opposite surface of the excitation coil. Sinusoidal excitation waves of 5 Hz and 25 Hz with amplitudes ranging from 0.25 V to 9 V were applied to samples with different amounts of the sigma phase, and the microstructures were characterized by scanning electron microscopy. The results show that the best testing condition consists of applying waves with amplitudes from 1 V to 2 V and using the first harmonic amplitude. Thus, the test proved effective for detecting the formation of the deleterious sigma phase and can follow the ability to absorb energy by impact and, thus, the material embrittlement.

Study and Characterization of Sigma Phase in Duplex Stainless Steel 2205 (03Kh22N6M2)

Study and Characterization of Sigma Phase in Duplex Stainless Steel 2205 (03Kh22N6M2)

Hot deformation behavior and microstructure evolution of each single phase in DSS

This paper presents a systematic experimental study to clarify the relationship between the hot deformation conditions and microstructure evolution and flow stress of the individual ferrite and austenite phases of duplex stainless steels (DSS), and to quantitatively compare the deformation behaviors of each phase under the same hot working conditions. Single-phase steels with the ferrite and austenite chemistry of the phases in the DSS were prepared for this investigation. The microstructure evolution and deformation behavior of the individual phases were investigated in one-pass hot uniaxial compression tests up to a strain of 0.8 at a wide range of working temperatures from 1000 to 1250 °C and strain rates from 0.1 to 30.0 s−1. The compressed microstructures of each phase were investigated, and microstructure evolution maps were proposed to demonstrate the relationship between the compressed microstructure and deformation conditions. The flow curves of each phase were also analyzed quantitatively, showing that the ferrite phase had lower hot strength and a lower work hardening exponent n, and higher strain rate sensitivity m and temperature sensitivity A compared to the austenite phase. The strength ratio of each phase from early to late deformation was expressed using the Zener-Hollomon parameter, and equations and constants for the strength ratio of the phases were proposed.

Developing a 2D finite element cutting model based on individual phases in orthogonal cutting of two-phase duplex stainless steel

This paper presents a novel method to model the metal cutting of 2205 duplex stainless steel alloy. A 2D finite element model was developed to predict the plastic behaviour of two phases of duplex stainless steel, austenite, and ferrite during orthogonal cutting. A mesh based on physical microstructure was created using mapping software OOF2. The finite model was setup on ABAQUS software based on the Lagrangian method. The cutting model allows for tracking of stress and strain on individual phases during chip formation. Simulated and experimental data showed good agreement in comparison of shear angle and cutting strain. Model was created to gain understanding of long-established issues in machining duplex stainless alloys. Future model development required to gain further insight on associated triggering mechanisms.

Micromechanical Observation and Numerical Simulation for Local Deformation Evolution of Duplex Stainless Steel.

The characteristics of local strain distribution and evolution of duplex stainless steel during the tensile process were studied using the digital image correlation (DIC) technique. In addition, the finite element inversion of nanoindentation experiments of austenitic and ferrite phases in duplex stainless steel was carried out to obtain the stress-strain response of the two phases. Further, based on the representative volume element (RVE) and the material parameters obtained from the finite element inversion method, the local stress and strain behavior of duplex stainless steel at microscale was simulated numerically. The results fit well with the experiments, showing that the austenite phase is softer than ferrite phase, with the larger strain zone concentrated in the austenite phase and the larger stress zone concentrated in the ferrite phase. The grain boundaries are prone to obvious stress and strain concentrations. The local stress and strain distributions are influenced by the shape and interaction of the grains, while the distribution features become more obvious as the load increases. The research results effectively reveal the two-phase interaction and local failure mechanism of duplex stainless steel, and may provide a reference for material preparation and safety design of related structures.

Real-time evolution and characterization of passive films on individual ferrite and austenite phases of duplex stainless steel

The passive film growth process, the composition of passive films, and the electrochemical behavior of ferrite and austenite individual phases on the unbroken surface of 2205 duplex stainless steel (DSS) were investigated by scanning electron microscopy, energy dispersive spectrometry, Kelvin probe force microscopy, magnetic force microscopy, in situ electrochemical atomic force microscopy, and Auger electron spectroscopy depth analysis. The electrochemical characteristics of the α/γ individual phases on the unbroken 2205 DSS surface were similar. The distribution of elements resulted in an element gradient at α/γ phase boundaries with a 2–6-μm width. Three stages of passive film formation were defined: nucleation, rapid growth, and stable growth. An even distribution of alloying elements with a difference in the concentration of Fe and Cr in the barrier layer was observed in the two-layer passive film of α/γ individual phases on the intact 2205 DSS surface. The preferential dissolution occurring on DSS was also discussed.

Investigation of Inhibitory Effects of Chitosan on Pitting and Electrochemical Corrosion Behavior Caused by Sigma Phase in Duplex Stainless Steels (DSS)

Investigation of Inhibitory Effects of Chitosan on Pitting and Electrochemical Corrosion Behavior Caused by Sigma Phase in Duplex Stainless Steels (DSS)

Strain hardening effect in austenite ferrite dual-phase steel subjected to tensile deformation

Duplex stainless steel (DSS) with an austenite-ferrite duplex microstructure offers high strength and toughness, excellent resistance to intergranular and chloride corrosion, and good weldability. The strain hardening behavior of austenite and ferrite phases in duplex stainless steel, when subjected to a tensile stress condition, is studied in the current work. Specimens were prepared from test blocks soaked at a temperature of 1160˚ C followed by furnace cooling to 1060 ˚C and water quenching. Heat-treated samples were deformed for varied percentage elongations. Microstructural analysis using optical microscopy revealed the deformations in austenite and ferrite phases. The concentrations of component alloying elements were determined using Energy Dispersive X-Ray Analysis (EDX) and area-scan techniques. At each level of percentage deformations, as well as on the fractured sample, microhardness measurements were taken on both austenite and ferrite phases. The hardness values of both austenite and ferrite phases were observed to rise continuously with an increase in deformation. The rise in hardness value of the ferrite phase was found to be significantly higher than the rise of hardness in the austenite phase, proving that the ferrite phase has a higher work hardening with deformation.

Microstructural and selective dissolution analysis of 316L austenitic stainless steel

Austenitic stainless steels (ASS) are among the most used structural materials because of their excellent mechanical and corrosive properties. In addition, 316L ASS has excellent weldability because of its low carbon content. This steel normally contains 5–10% residual delta ferrite to avoid hot cracking and micro-fissuring of the weld metal. Many studies show that selective dissolution of austenite or ferrite can occur in 316L steel. In a survey of the selective dissolution of phases in duplex stainless steel (DSS), Lo and coauthors studied the effect of electrolyte composition (H2SO4 + HCl solutions) on the active–passive transition. They showed that corrosion occurred only in the austenitic phase in solutions of 2M H2SO4 + 0.25–1.2M of HCl. Increasing the % of HCl, > 1.2%, austenite, and ferrite contributed to the corrosion process. The present authors did not find the effect of this electrolyte in the study of selective corrosion in 316L ASS welded in the literature. Therefore, this study aims to determine how the phases behave on 316L ASS by applying 2M H2SO4 + 1.5M HCl solution. In this work, we welded the steel by the autogenous TIG process using three heat inputs. The purpose of welding was to obtain different amounts and morphology of ferrite and determine whether this affected the selective dissolution of the phases. After the corrosion tests, the samples showed selective austenite corrosion irrespective of the volume fraction and morphology of the ferrite. In summary, the corrosive etches severely consumed the austenite because of the significant difference in the electrochemical potential between the phases. Therefore, with more nickel, austenite functioned as an anodic region, and the δ-ferrite, with more chromium, worked as a cathodic region.

Low cycle fatigue analysis of heat treated duplex stainless steel

ABSTRACT Duplex stainless steels (DSS) have become more attractive than single-phase austenitic and ferrite grades in industrial application due to their high strength and corrosion resistance in chloride-containing media. Due to low ductility region under hot working conditions these materials require special attention during processing. As heat treatments influence low cycle fatigue characterisation. The characteristics and properties of the industrially employed duplex grades has been discussed, moreover the presence and precipitation of sigma phases has been highlighted through ageing treatments. Based on up to date achieved outcomes, it appears that a quite homogeneous and good mechanical properties can be obtained by controlled composition and heat treatments .and it also enhances the wear properties. The most commonly formed and observed pernicious phase in DSS is sigma phase particles precipitation in a short period of time over a wide range of temperature. Hence an attempt has been made in our investigation to evaluate the low cycle fatigue changes when they are subjected to temperature 1100°C for a shorter duration for about 60 minutes. The low-cycle fatigue behaviour of a DSS was comparatively studied in standard heat-treated and aged at 800°C for 4 h.

Dilatometric Cycles in the Study of Precipitation of Intermetallic Phases in Duplex Stainless Steels

Duplex stainless steels (DSS) are used in off-shore platforms due to their good properties resulting from the combination of two phases, namely austenite and ferrite. Secondary intermetallic phases, such as alpha prime (α´) and sigma (σ), which are harmful, can be formed at temperatures above 400 °C, or by a casting process. This study investigates the formation of sigma phase by thermal cycles running in a dilatometer (800 °C, 850 °C, 900 °C and 1,000 °C) for 1h, and at 850 °C for 3h and 7h. The optical microscopy of the DSS microstructure subjected to 800, 850 and 900 °C for one hour revealed a small fraction of σ phase with nucleation in the contour of ferrite/austenite phases, and tests by ferritoscope indicated a decrease (~4%) in a magnetic fraction (ferrite). However, samples subjected to dilatometric cycles at 850 °C for 3 hours showed an 18% decrease in the magnetic phase and quantitative metallography revealed a 33% fraction of σ phase precipitated in the ferrite phase. Tests conducted at 850 ºC for 7 hours indicated a high fraction of σ phase precipitated (approximately 50%), starting in the grain boundary, austenite/ferrite, and advancing in ferrite grain.

Rapid precipitation of intermetallic phases during isothermal treatment of duplex stainless steel joints produced by friction stir welding

The propensity of duplex stainless steels to form secondary phases that harm their main properties is a metallurgical problem that continues to be studied. Friction stir welding has shown excellent results when applied to duplex stainless steels, enabling the formation of joints that maintain the austenite-ferrite balance, without the presence of secondary phases. Welding processes cause significant microstructural reformulation in the joint microstructure, so the properties and metallurgical behavior of welded joints differ significantly from those of the corresponding base metals. The formation of secondary phases in duplex stainless steels occurs by diffusional transformation processes that are dependent on the transformation time, with the kinetics being influenced by the previous microstructure. Although friction stir welding makes it possible to obtain joints free from secondary phases, it is important to study the propensity of welded joints to subsequently form secondary phases, in order to be able to understand the behavior of these joints when they are subjected to hot operations, such as weld repairs. In this work, friction stir welded joints were treated isothermally at 850 °C and were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, Ferritoscope measurements, and X-ray diffractometry. The results revealed that the friction stir welded joints presented much faster formation of intermetallic phases, compared to the base metals, with the appearance of regions showing advanced stages of intermetallic phase precipitation after only 5 min exposure at 850 °C.

Modeling of Effects of Temperature and Alloying Elements on Austenite Phase Growth Rate in Duplex Stainless Steel

The growth rate of austenite phase from overcooled ferrite phase in duplex stainless steel was investigated to clarify the quantitative effects of temperature and chemistry in isothermal heating process. The proper fraction of ferrite and austenite phase is very important to obtain the maximum performance such as toughness and corrosion resistance of the steels. However, the fraction in heat affected zone (HAZ) in weldments can be changed during welding process.

Micro-Structural Characterization and Semi Quantitative Chemical Analysis of Ferrite and Austenite Phases in Duplex Stainless Steels

The PREN value of the alloy may lead to a misleading prediction of pitting corrosion resistance because in the event of an imbalance of δ ferrite and austenite volume fraction, by Quantitative metallographic analyzes. In this work three laminated duplex stainless steels with different PRENs were characterized: UNS S32304 (Lean Duplex), UNS S31803 (Duplex) and UNS S32750 (Super Duplex), where several techniques of microstructural analysis were used, such as optical emission spectrometry, optical microscopy and scanning electron microscopy/EDS. For all the alloys studied, it was observed that the δ ferrite phase presents higher Cr and Mo contents than the austenite phase, which in turn had Ni contents higher than the δ ferrite. Therefore we can observe that the PRENN values for the Super Duplex alloy were generally higher, as well as in the ferritic and austenitic phase, at a level of 12.25% in relation to Duplex, and more than 50% higher than Lean Duplex, which leads to the definition of the alloy having greater resistance to pitting corrosion.

Characterization of Native Oxide and Passive Film on Austenite/Ferrite Phases of Duplex Stainless Steel Using Synchrotron HAXPEEM

A new measurement protocol was used for microscopic chemical analysis of surface oxide films with lateral resolution of 1 μm. The native air-formed oxide and an anodic passive film on austenite and ferrite phases of a 25Cr-7Ni super duplex stainless steel were investigated using synchrotron hard X-ray photoemission electron microscopy (HAXPEEM). Pre-deposited Pt-markers, in combination with electron backscattering diffraction mapping (EBSD), allowed analysis of the native oxide on individual grains of the two phases and the passive film formed on the same area after electrochemical polarization of the sample. The results showed a certain difference in the composition of the surface films between the two phases. For the grains with (001) crystallographic face // sample surface, the native oxide film on the ferrite contained more Cr oxide than the austenite. Anodic polarization up to 1000 mV/Ag/AgCl in 1M NaCl solution at room temperature resulted in a growth of the Cr- and Fe-oxides, diminish of Cr-hydroxide, and an increased proportion of Fe3+ species.

A New methodology of Nondestructive Testing for Quantitative Evaluation of Sigma Phase in Duplex Stainless Steels

Duplex Stainless Steels (DSSs) are widely used in the oil and gas industry. When the steel is subjected to a thermal gradient, as in welding may occur precipitation of intermetallic phases. Among these phases, there is the sigma phase, which even in small quantities reduces drastically the mechanical and corrosion resistance properties. According to some studies in the literature, the limit amount of sigma phase present in steel in welded joints must be in a maximum volume fraction of 2,5%. In search to improve the detection sensitivity of the sigma phase, this paper presents results obtained by Linear Sweep Voltammetry (LSV) in Duplex Stainless Steel UNS S31803 as a Nondestructive Testing (NDT) model. The innovation in the application of this test is to use a microcell-based system that allows a reduced area of ​​analysis obtaining density of currents in the order of microamperes and in the use of selective electrolytes to the elements of the oxides present in the intermetallic phases. With the use of these systems and through the optimization of control parameters it was possible to detect very low amounts of deleterious phases such as the sigma and chi phases.

Micro-deformation evolutions of the constituent phases in duplex stainless steel during cyclic nanoindentation

Cyclic elastoplastic deformation behaviors of austenite phase and ferrite phase in a duplex stainless steel were investigate by load-controlled cyclic nanoindentation with a Berkovich indenter. During the tests, the maximum penetration depth per cycle increased rapidly with cycle number at transient state, and reached stable at quasi-steady state. Plastic dissipated energy was quantitatively proved to be the driving force for the propagation of deformation zones during cyclic nanoindentation tests. Transmission electron microscopy combined with FIB was used to reveal the deformation mechanisms of both phases underneath indents with cycles. After quasi-static single loading, nucleation and concentration of dislocations were observed in both austenite phase and ferrite phase under the indenter. After cyclic loading, dislocations propagated to further regions in both phases. Besides, slip bands were generated within single nanoindentation and propagated during the subsequent cyclic nanoindentation. The sizes of the deformation regions for both phases under the indents after cyclic indentation observed by TEM were consistent with those calculated by the expansion model of spherical cavity.

Double-loop electrochemical potentiokinetic reactivation behaviour of continuously cooled SUS329J4L duplex stainless steel

ABSTRACTSigma phase generally precipitates in duplex stainless steel during slow cooling, which usually causes loss in both corrosion resistance and fracture toughness. To develop a non-destructive method to detect the sigma phase in duplex stainless steel for in situ use in the quality management of large structures, the double-loop electrochemical potentiokinetic reactivation behaviour of continuously cooled duplex stainless steel SUS329J4L at different cooling rates was investigated under different conditions. The polarisation condition with the solution of 0.5 M H2SO4 + 0.01 M KSCN + 0.5 M NaCl and the potential scanning rate of 0.83 mV s−1 were determined as suitable and sensitive.This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).

Effect of Mo on interaction between α/γ phases of duplex stainless steel

The role of molybdenum in the interaction of the composed phases of 2205 duplex stainless steel (DSS) in the 3.5-wt% NaCl solution was investigated, by comparing the untreated 2205 DSS sample with two single-austenite and single-ferrite samples prepared via selective dissolution. The results demonstrate that, the interaction is beneficial to the whole passive film properties of 2205 DSS by promoting both the γ-phase and α-phase. The passivity of 2205 DSS at high potential strongly depends on the protective ability of Mo species. Compared with α-phase and γ-phase, Mo species of the 2205 DSS are more capable of keeping stable and being beneficial to passive film recovery, which make 2205 DSS show enhanced performance in pitting resistance and repassivation.

INFLUENCE OF ANNEALING ON THE DEPTH MICROSTRUCTURE OF THE SHOT PEENED DUPLEX STAINLESS STEEL AT ELEVATED TEMPERATURE

The depth profiles of residual stresses and lattice parameters in the surface layers of shot peened duplex stainless steel at elevated temperature were investigated utilizing X-ray diffraction analysis. At each deformation depth, residual stress distributions in both ferrite and austenite were studied by X-ray diffraction stress analysis which is performed on the basis of the sin[Formula: see text] method and the lattice parameters were explored by Rietveld method. The results reveal that difference changes of depth residual compressive stress profiles between ferrite and austenite under the same annealing condition are resulted from the diverse coefficient of thermal expansion, dislocation density, etc. for different phases in duplex stainless steel. The relaxations of depth residual stresses in austenite are more obvious than those in ferrite. The lattice parameters decrease in the surface layer with the extending of annealing time, however, they increase along the depth after annealing for 16[Formula: see text]min. The change of the depth lattice parameters can be ascribed to both thermal expansion and the relaxation of residual stress. The different changes of microstructure at elevated temperature between ferrite and austenite are discussed.